To guide and condition the emitted light, use is made for example of mirrors referred to as facet mirrors, which generally have a plurality of closely adjacent, relatively small-area mirror facets or micromirror arrays. As regards their orientation, the mirror facets are usually controllable individually or in groups, as are the micromirror arrays. They are generally arranged in recesses in a for example plate-type, flat or curved mirror carrier. During operation of the associated projection exposure apparatus, the mirrors used are exposed to high thermal loads and have to be actively cooled. The thermal loads can stem from the incident electromagnetic, high-energy radiation and also—in particular when micromirror arrays are used—from electrical power loss of electronic components arranged directly beneath the mirror. The heat that arises is discharged usually into the actively cooled mirror carrier.
During operation of the apparatus, the distribution of the illumination intensity and thus the thermal load over the entire surface of the facet mirror often changes. Examples of causes for this that may be mentioned are for example the change in illumination settings (i.e. the intensity distribution of the electromagnetic radiation on the reticle) or the switching on and off of the light source between two wafers or two batches. This change in the thermal load often results in a deformation, in particular in bending of the mirror carrier, this being typically reflected in a change in the orientation of the mirror surfaces and thus in reduced imaging quality of the apparatus as a whole. This can occur in particular because the thermal load arises, on account of the system, on the illuminated side of the mirror arrangement.
In order to compensate for an asymmetric energy input at a mirror arrangement and to reduced thermally induced deformations, WO2012126830A1 suggests providing the mirror arrangement with at least two separate cooling circuits, such that the optically active surface of the mirror arrangement can be cooled more in at least one subregion than in a further subregion. Such a configuration with several cooling circuits is relatively complicated in terms of production and operation, however.